.TH std::reference_wrapper 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::reference_wrapper \- std::reference_wrapper

.SH Synopsis
   Defined in header <functional>
   template< class T >             \fI(since C++11)\fP
   class reference_wrapper;

   std::reference_wrapper is a class template that wraps a reference in a copyable,
   assignable object.

   Specifically, std::reference_wrapper is a CopyConstructible and CopyAssignable
   wrapper around a reference to object or reference to function of type T. Instances
   of std::reference_wrapper are objects (they can be copied or stored in containers)
   but they are implicitly convertible to T&, so that they can be used as arguments
   with the functions that take the underlying type by reference.

   If the stored reference is Callable, std::reference_wrapper is callable with the
   same arguments.

   Helper functions std::ref and std::cref are often used to generate
   std::reference_wrapper objects.

   std::reference_wrapper is used to pass objects by reference to std::bind, the
   constructor of std::thread, or the helper functions std::make_pair and
   std::make_tuple. It can also be used as a mechanism to store references inside
   standard containers (like std::vector) that cannot normally hold references.

   std::reference_wrapper is guaranteed to be TriviallyCopyable. \fI(since C++17)\fP

   T may be an incomplete type. \fI(since C++20)\fP

.SH Member types

   type                             definition
   type                             T
   result_type                      The return type of T if T is a function. Otherwise,
   (deprecated in C++17)(removed in not defined
   C++20)
                                      * if T is a function or pointer to function that
                                        takes one argument of type A1, then
                                        argument_type is A1
   argument_type                      * if T is a pointer to member function of class
   (deprecated in C++17)(removed in     T0 that takes no arguments, then argument_type
   C++20)                               is T0*, possibly cv-qualified
                                      * if T is a class type with a member type
                                        T::argument_type, then argument_type is an
                                        alias of that
                                      * if T is a function or pointer to function that
                                        takes two arguments of types A1 and A2, then
                                        first_argument_type is A1
   first_argument_type                * if T is a pointer to member function of class
   (deprecated in C++17)(removed in     T0 that takes one argument, then
   C++20)                               first_argument_type is T0*, possibly
                                        cv-qualified
                                      * if T is a class type with a member type
                                        T::first_argument_type, then
                                        first_argument_type is an alias of that
                                      * if T is a function or pointer to function that
                                        takes two arguments of type s A1 and A2, then
                                        second_argument_type is A2
   second_argument_type               * if T is a pointer to member function of class
   (deprecated in C++17)(removed in     T0 that takes one argument A1, then
   C++20)                               second_argument_type is A1, possibly
                                        cv-qualified
                                      * if T is a class type with a member type
                                        T::second_argument_type, then
                                        second_argument_type is an alias of that

.SH Member functions

   constructor   stores a reference in a new std::reference_wrapper object
                 \fI(public member function)\fP
   operator=     rebinds a std::reference_wrapper
                 \fI(public member function)\fP
   get           accesses the stored reference
   operator T&   \fI(public member function)\fP
   operator()    calls the stored function
                 \fI(public member function)\fP

.SH Non-member functions

   operator==  compares reference_wrapper objects as their stored references
   operator<=> \fI(function)\fP
   (C++26)

   Deduction guides\fI(since C++17)\fP

.SH Helper classes

                                                       determines the common reference
   std::basic_common_reference<std::reference_wrapper> type of reference_wrapper and
   (C++23)                                             non-reference_wrapper
                                                       \fI(class template specialization)\fP


.SH Possible implementation

   namespace detail
   {
       template<class T> constexpr T& FUN(T& t) noexcept { return t; }
       template<class T> void FUN(T&&) = delete;
   }

   template<class T>
   class reference_wrapper
   {
   public:
       // types
       using type = T;

       // construct/copy/destroy
       template<class U, class = decltype(
           detail::FUN<T>(std::declval<U>()),
           std::enable_if_t<!std::is_same_v<reference_wrapper, std::remove_cvref_t<U>>>()
       )>
       constexpr reference_wrapper(U&& u)
           noexcept(noexcept(detail::FUN<T>(std::forward<U>(u))))
           : _ptr(std::addressof(detail::FUN<T>(std::forward<U>(u)))) {}

       reference_wrapper(const reference_wrapper&) noexcept = default;

       // assignment
       reference_wrapper& operator=(const reference_wrapper& x) noexcept = default;

       // access
       constexpr operator T& () const noexcept { return *_ptr; }
       constexpr T& get() const noexcept { return *_ptr; }

       template<class... ArgTypes>
       constexpr std::invoke_result_t<T&, ArgTypes...>
           operator() (ArgTypes&&... args ) const
               noexcept(std::is_nothrow_invocable_v<T&, ArgTypes...>)
       {
           return std::invoke(get(), std::forward<ArgTypes>(args)...);
       }

   private:
       T* _ptr;
   };

   // deduction guides
   template<class T>
   reference_wrapper(T&) -> reference_wrapper<T>;

.SH Example

   Demonstrates the use of std::reference_wrapper as a container of references, which
   makes it possible to access the same container using multiple indices.


// Run this code

 #include <algorithm>
 #include <functional>
 #include <iostream>
 #include <list>
 #include <numeric>
 #include <random>
 #include <vector>

 void println(auto const rem, std::ranges::range auto const& v)
 {
     for (std::cout << rem; auto const& e : v)
         std::cout << e << ' ';
     std::cout << '\\n';
 }

 int main()
 {
     std::list<int> l(10);
     std::iota(l.begin(), l.end(), -4);

     // can't use shuffle on a list (requires random access), but can use it on a vector
     std::vector<std::reference_wrapper<int>> v(l.begin(), l.end());

     std::ranges::shuffle(v, std::mt19937{std::random_device{}()});

     println("Contents of the list: ", l);
     println("Contents of the list, as seen through a shuffled vector: ", v);

     std::cout << "Doubling the values in the initial list...\\n";
     std::ranges::for_each(l, [](int& i) { i *= 2; });

     println("Contents of the list, as seen through a shuffled vector: ", v);
 }

.SH Possible output:

 Contents of the list: -4 -3 -2 -1 0 1 2 3 4 5
 Contents of the list, as seen through a shuffled vector: -1 2 -2 1 5 0 3 -3 -4 4
 Doubling the values in the initial list...
 Contents of the list, as seen through a shuffled vector: -2 4 -4 2 10 0 6 -6 -8 8

.SH See also

   ref              creates a std::reference_wrapper with a type deduced from its
   cref             argument
   \fI(C++11)\fP          \fI(function template)\fP
   \fI(C++11)\fP
   bind             binds one or more arguments to a function object
   \fI(C++11)\fP          \fI(function template)\fP
   unwrap_reference
   unwrap_ref_decay get the reference type wrapped in std::reference_wrapper
   (C++20)          \fI(class template)\fP
   (C++20)
